The invention relates generally to the area of DC motor drives, and specifically the invention provides a unique circuit configuration for a single-phase full-wave DC motor drive.
Typically, in a DC motor drive circuit, an AC supply is rectified by thyristors whose conduction angle is controlled so as to control the motor armature current thereby controlling some factor such as speed or torque. For full-wave operation, two thyristors are necessary; and for a reversible motor, the number is doubled. The term thyristor as used throughout this specification and in the claims shall mean any type of controlled rectifier, e.g. silicon controlled rectifiers, gate control switches, mutlilayer two-terminal devices, etc.
A typical drive circuit is illustrated in FIG. 1. Four thyristors TH1-TH4 are connected in a bridge circuit which is energized across one diagonal by the secondary winding 10 of a supply transformer. The primary winding 11 being connected to terminals 12 for energization from the main power supply. The gate electrodes of the thyristors are omitted for simplicity. Across the other diagonal of the bridge is connected a choke shown for convenience as two inductors L1 and L2 whose junction constitutes a center tap connected to one motor terminal 13. The other motor terminal 14 is connected to the center tap on the secondary winding 10.
The choke is necessary for various reasons. Apart from improving the form factor and helping, by its leakage impedance, to limit fault currents which could demagnetize permanent magnet motors, it has the essential function of providing a back EMF when the motor is under no-load or virtually no-load conditions. In this connection, it is well-known that when the motor is at a standstill, it is necessary to fire the thyristors over a small conduction angle to keep the circuit ready to respond immediately to a command signal to move or produce a torque in either direction. This would produce very large no-load currents in the absence of the choke. It follows that forward drive is achieved by turning on thyristor TH1 in positive half cycles and thyristor TH3 in negative half cycles. Reverse drive is achieved by turning on thyristor TH4 in positive half cycles and thyristor TH2 in negative half cycles. In this bridge configuration, only one-half of the choke is used in either direction of movement. This limits the power which can be dissipated in the choke and tends to cause DC polarization of the choke. Further, the choke is used inefficiently in that it can only dissipate half the power that it should be able to for its size.
Another disadvantage of the bridge circuit is that all four thyristors have to have separate protective circuits. For simplicity only the protective circuit for thyristor TH3 is shown as a capacitor 15 and resistor 16 in parallel with the thyristor.
The conduction angle is normally determined by adjusting the firing angle in each half cycle within the range of 0.degree. to 180.degree.. The thyristor turns off at 180.degree. with a reactive load because there is no forward current. As will be appreciated by those who are skilled in the art, the conduction angle becomes smaller as the firing angle occurs later. The firing angle is determined by comparing a ramp signal with a comparator input signal. A problem with known circuits is that they use two ramp generators, one for the positive half cycle and one for the negative half cycle. This makes it difficult to insure uniformity of operation between positive and negative half cycles.
Further, prior art circuits control the conduction angle as a linear function of the level of an input signal. Because of the sinusoidal shape of the AC input signal, the mean output voltage is not a linear function of conduction angle but is proportional to the cosine of the conduction angle.
The DC motor drive circuit disclosed herein is effective to overcome the disadvantages recited above.